Figure 1. Five models for the development of abyssal hills. a, back-tilted fault blocks (episodic inward-dipping normal faulting off-axis), b, horst and graben (episodic inward- and outward-dipping faulting off-axis), c, whole volcanoes (episodic volcanism on-axis), d, split volcanoes (episodic volcanism and splitting on-axis), e, horsts bounded by inward-dipping normal faults and outward-dipping volcanic growth faults (episodic faulting off-axis and episodic volcanism on or near-axis).